Anand, M.

Barnes, J. J.

Barrett, T. J.

Charlier, B. L. A.

Franchi, I. A.

Grady, M. M.

Greenwood, R. C.

English

Open Research Online (The Open University)

Open Research OnlineThe Open University’s repository of research publicationsand other research outputsChlorine isotope variation in eucritesConference or Workshop ItemHow to cite:Barrett, T. J.; Barnes, J. J.; Anand, M.; Franchi, I. A.; Greenwood, R. C.; Charlier, B. L. A. and Grady, M.M. (2016). Chlorine isotope variation in eucrites. In: 79th Annual Meeting of the Meteoritical Society, 7-12 Aug2016, Berlin, Germany.For guidance on citations see FAQs.c© [not recorded]Version: Version of RecordCopyright and Moral Rights for the articles on this site are retained by the individual authors and/or other copyrightowners. For more information on Open Research Online’s data policy on reuse of materials please consult the policiespage.oro.open.ac.ukCHLORINE ISOTOPE VARIATION IN EUCRITES. T. J. Barrett1, J. J. Barnes1, M. Anand1, 2, I. A. Franchi1, R. C. Greenwood1, B. L. A. Charlier1, 3, M. M. Grady1.  1Department of Physical Sciences, The Open University, Walton Hall, Milton Keynes, MK7 6AA, U.K. E-mail: thomas.barrett@open.ac.uk. 2Earth Sciences Department, Natural History Museum, London, SW7 5BD, U.K.  3Victoria University of Wellington, Wellington 6140, N.Z.  Introduction: In general, Solar System materials exhibit a relatively narrow range in bulk Cl isotope composi-tions, with terrestrial rocks and martian and chondritic meteorites having δ37C values of  ~ -4 to ~ +1.8 ‰ [1-3]. However, lunar rocks are an important exception, displaying a significant range in their bulk and apatite δ37Cl values from – 4 to +81 ‰ [4-10]. Howardites, Eucrites and Diogenites (HEDs) represent the largest suite of crustal and sub-crustal rocks available from a differentiated basaltic asteroid (most probably 4 Vesta) [11, 12] and are also some of the oldest igneous rocks in the Solar System [13, 14]. Therefore, understanding the volatile inventories of the eucrites can help us to constrain the processes that operated on asteroid parent bodies early in Solar System history. In order to investigate the history of volatiles in eucrites, we have measured the Cl isotopic composition of apatite from five eucrites (Dar al Gani (DaG) 844, DaG 945, Millbillillie, Moore County and Stannern), which experienced a wide range of petrological and geochemical histories. Apatite grains from these five samples were previously analysed for the H isotopic composition of the OH they contain [15]. Methods: Apatite analyses were performed using the Cameca NanoSIMS 50L instrument at The Open University, using a modified protocol reported in Tartèse et al. [8] and Barnes et al. [9]. Typically ~ 5 × 5 µm raster areas were analysed, however for smaller grains this area was reduced to ~ 3 × 3 µm. Results: A total of 19 measurements from 15 individual apatite grains from the five eucrites were obtained. Apatite Cl content ranged from ~25 ppm Cl in DaG 945 to ~2030 ppm Cl in Millbillillie. Considerable intra-grain variability in Cl abundance is observed in Moore County, with a single grain having a range of ~360 ppm. However, apatite crystals from most samples display more restricted variation (both inter as well as intra-grain). The δ37Cl values ob-tained for apatite from four of the five samples are within error of bulk terrestrial and chondritic values. However, two apatite grains in Moore County display more enriched compositions (~ +6.6 ± 3.4 ‰ and +13.1 ± 3.0 ‰) than terres-trial values. In contrast to all the other eucritic samples examined in this study, DaG 945 apatite show a very heavy δ37Cl composition, with values ranging from ~ +27.2 ± 4.4 ‰ to ~ +35.6 ± 6.2 ‰, which incidentally are some of the highest measured values among the Solar System materials. Discussion: In all eucrite samples, apatites display consistent δD values [15], with no correlation between δ37Cl and δD values. The data presented in this study appear to display a negative correlation between δ37Cl and Cl content, an observation consistent with the degassing model for the Moon proposed by Sharp et al.[4]. The petrological history of DaG 945, however, is very different to a degassed lunar basalt [16]. DaG 945 is a residual eucrite which is believed to have experienced prolonged heating at temperatures of ~ 800 – 1000 °C and also partial melt extraction [16]. It is possible that the extreme metamorphism which DaG 945 experienced may have caused the large fractionation in Cl isotopes. Alternative hypotheses used to explain the extreme fractionations in Cl observed in lunar samples, predict a correlation between bulk-rock trace-elements and δ37Cl values [6, 9]. Whilst such a correlation was observed by Schneck et al. [17], such a correlation is not apparent in our dataset. In fact, DaG 945 shows a depletion in its chondrite-normalised La/Yb ratio compared to other eucrite samples, following a trend opposite to that predicted by lunar models [6, 9]. Conclusion: Our results include some of the heaviest chlorine isotopic compositions recorded so far in the Solar System. The data suggest that the Cl isotope system is complex and Cl may fractionate differently under different magmatic conditions, with potentially multiple processes at work. In order to understand the process(s) responsible for producing the large fractionation in Cl isotopes observed in the residual eucrite DaG 945, additional residual eu-crites with similar petrological characteristics will need to be analysed.  References: [1] Sharp Z. D. et al. 2007 Nature 466:1062-1065. [2] Sharp Z. D. et al 2013 Geochimica et Cosmo-chimica Acta 107:189-204. [3] Williams J. T. 2016. In press Meteoritics & Planetary Science. [4] Sharp Z. D. et al. 2010. Science 329:1050-1053. [5] Wang Y. et al. 2012. 75th Meteoritical Society Conference Abstract #5170. [6] Boyce J. W. et al. 2015 Science Advances 1. [7] Treiman A. H. et al 2014. American Mineralogist 99:1860-1870. [8] Tartèse R. et al. 2014. Meteoritics & Planetary Science 49:2266–2289. [9] Barnes J. J. et al. 2016. In press in Earth and Planetary Science Letters. [10] Robinson K. L. 2016. 47th Lunar and Planetary Science Conference Abstract #2199. [11] McCord T. B. et al. 1970 Science 168:1445-1447. [12] De Sanctis M. C. et al. 2012 Science 366:697-700. [13] Misawa K. et al. 2005. Geochimica et Cosmochimica Acta 69:5847-5861. [14] Zhou Q. et al. 2013. Geochimica et Cosmochimica Acta 110:152-175. [15] Barrett T. J. et al. 2016 In press Meteoritics & Planetary Science. [16] Yamaguchi A.et al. 2009. Geochimica et Cosmochimica Acta 73:7262-7182. [17] Schneck U. G. et al. 2016 47th Lunar and Planetary Science Conference Abstract #2978.  6517.pdf79th Annual Meeting of the Meteoritical Society (2016)

Chlorine isotope variation in eucrites

Chlorine isotope variation in eucrites

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